As is known in the art, epitaxial growth of semiconductor films, such as performed by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) depends critically on the magnitudes and uniformities of reactant atoms on the substrate surface as well as the temperature and its uniformity across the substrate. In the specific example of plasma MBE growth of GaN, gallium atoms are evaporated onto the substrate surface in the presence of reactive nitrogen provided by a plasma source. The highest quality films are obtained when the net gallium beam flux (incoming gallium flux minus the gallium desorption rate) exceeds the reactive nitrogen beam flux resulting in a gallium adlayer on the growing surface. The gallium desorption rate from the surface is exponentially dependent on substrate temperature. If the adlayer does not exist, the surface will roughen with degraded material properties and will be visible as surface haze. When the net gallium beam flux exceeds the reactive nitrogen flux, an adlayer forms and begins to thicken, creating small gallium balls. Initially these small gallium balls are not an issue for material growth. If the gallium balls become too large, however, they will back-dissolve the substrate surface, resulting in surface roughening and will also interfere with the growth process. Therefore during growth the large gallium balls are periodically removed by interrupting the gallium and nitrogen beams, resulting in the large gallium balls to be desorbed from the substrate surface. Consequently at a given substrate temperature there is a growth window for gallium fluxes and growth times to obtain smooth surfaces without the formation of haze or large gallium balls. Surface haze can also form if the substrate surface is not atomically clean prior to growth since the surface crystallinity is disrupted leading to a rough surface.
Another issue encountered in epitaxial film growth on substrates requiring a backside coating for effective and uniform heating is the presence of pinholes in the coating. The occurrence of pinholes leads to substrate temperature non-uniformities in the vicinity of the pinhole and can perturb pyrometer readings.
One current technique for in situ monitoring the substrate surface for GaAs and GaN growth is RHEED (reflection high energy electron diffraction). In this technique, a focused high energy electron beam tangentially impinges on the substrate surface creating an electron diffraction pattern sensitive to surface stoichiometry. The RHEED intensity can also be used to monitor the build-up of evaporants on the surface. However, the inventors have recognized that the technique has important limitations. First, the RHEED beam samples only several square millimeters of a substrate surface that may be several inches in diameter. Consequently the impact of flux non-uniformities or temperature non-uniformities across the substrate is not characterized by RHEED. Growth conditions may be adjusted correctly inside the RHEED measurement area, but the conditions outside the probe area are unknown. This limitation becomes more serious with increasing wafer diameter not only due to the limited sample area but also the increased non-uniformity of film fluxes and substrate temperatures over larger areas. The inventors have also recognized that a second concern is that the RHEED high energy electron beam (typically 10-20 KeV) impinging on the substrate surface may alter the growth in this region. As a practical matter, the surface is monitored with RHEED less than 10% of the time to minimize any effect of the RHEED beam on material growth.
Other techniques are also available to provide in situ information of the growing film. One technique uses pyrometry to measure the substrate temperature. However the measurement area is less than 10% of the surface area of a 3-inch wafer. Ellipsometry has been used in other laboratories to provide in situ information. However its measurement area is again less than 10% of a 3-inch wafer.
The inventors have recognized that due to non-uniformities in beam fluxes and substrate temperature, a technique is needed to monitor the entire wafer for substrate scratches and polish damage, surface haze, metallic accumulations such as gallium balls, pinholes in metal coatings, and other macroscopic defects. Further a technique is needed to monitor wafers that are rotated to improve non-uniformities in beam fluxes and substrate temperatures. With such a technique, large gallium balls could be periodically eliminated during growth by adjusting growth conditions. Thus, GaN growth can be performed with excess gallium on the surface without the consequences of large gallium balls affecting the growth. Further, knowledge of the location and the appearance of haze during a run may assist in diagnosing the problem with the growth conditions and/or substrate preparation. The location and concentration of pinholes or other macroscopic defects also need to be characterized. Furthermore the relative temperature uniformity across the wafer can be determined as cold spots (reduced gallium desorption) lead to gallium balls and hot spots (enhanced gallium desorption) lead to haze.
Thus, in accordance with the invention, apparatus and method are provided for growing and observing the growth of epitaxial layers on a wafer. The apparatus includes: epitaxial growth apparatus; a source of light mounted to illuminate an entire surface of the wafer in the apparatus during growth of the epitaxial layer on the entire surface of the wafer; and apparatus for observing scattering of the light from the entire surface of the wafer during growth of the epitaxial layer on the entire surface of the wafer. The apparatus is suitable for stationary or rotating wafers.
The method includes growing the epitaxial layer on a surface of the wafer and observing the light scattered from the entire surface of the wafer during growth of the epitaxial layer on the entire surface of the wafer. The growing process is varied in accordance with the observation.
In one embodiment, the epitaxial layer is gallium nitride (GaN) and the entire surface of the wafer is observed for balls of gallium.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.